Using a suite of microwave remote sensing instruments
aboard satellites, scientists at NASA’s Jet Propulsion
Laboratory (JPL), Pasadena, Calif., and the University of
Montana, Missoula, have observed a recent trend of earlier
thawing across the northern high latitudes.

This regional thawing trend, advancing almost one day a year
since 1988, has the potential to alter the cycle of
atmospheric carbon dioxide intake and release by vegetation
and soils across the region, potentially resulting in changes
in Earth’s climate. The lengthening growing season appears to
be promoting more carbon uptake by the vegetation than is
being released into the atmosphere for the region. How long
this trend will occur depends on whether soils continue to
remain cold and wet.

Research scientists have been studying freeze/thaw dynamics in
North America and Eurasia’s boreal forests and tundra to
decipher effects on the timing and length of the growing
season. These regions encompass almost 30 percent of global
land area. They store a major portion of Earth’s carbon in
vegetation, in seasonally frozen and permafrost soils. Large
expanses of boreal forest and tundra are underlain by
permafrost, a layer of permanently frozen soil found
underneath the active, seasonally thawed soil.

“Frozen soil can store carbon for hundreds to thousands of
years,” said lead author Dr. Kyle McDonald of JPL, “but when
the permafrost thaws and begins to dry out, it releases the
carbon back into the atmosphere.” The concern is that
eventually carbon released from the soil will prevail over the
amount being taken in by growing plants. Carbon dioxide levels
in the atmosphere would increase at an accelerated rate,
fostering even greater warming of the region and affecting
global climate.

With help from NASA radars and other orbiting satellite
microwave remote sensing instruments, including the National
Oceanic and Atmospheric Administration’s Special Scanning
Microwave/Imager, scientists can monitor growing season
dynamics of the global boreal forest and tundra daily. These
instruments sense the electrical properties of water in the
landscape, allowing scientists to determine exactly when and
where the springtime thaw occurs.

Because of the large extent and location of boreal forest and
tundra, and the global reservoir of carbon stored in their
vegetation and soils, this region is extremely sensitive to
environmental change. It has the capacity to dramatically
impact Earth’s climate.

“If global climate change is happening, here’s where you would
expect to see it first,” McDonald said.

As the research team observed, the earlier the spring thaw
occurs, the longer the growing season. These changes appear to
be promoting plant growth for the region. The longer growing
season allows plants to remove more carbon dioxide from the
atmosphere over a longer period of time.

Carbon dioxide is an important greenhouse gas that, if left in
the atmosphere, would promote additional warming. The plants
release oxygen and store the carbon as biomass that eventually
decomposes and transfers the carbon into the soil. Soil
microbes decompose dead plant material, returning a portion of
the soil carbon back into the atmosphere. The rate which soil
microbes decompose plant material and release carbon to the
atmosphere is also very sensitive to temperature. It could
potentially increase with warming temperatures and longer
growing seasons.

From this general study, McDonald, Dr. John Kimball of the
University of Montana, and JPL’s Erika Podest have lead three
different investigations, each focusing on different
noticeable changes in the boreal region. Results of three
related papers on this research will be presented to the
American Geophysical Union’s Fall Meeting this week in San

The research is funded by NASA’s Earth Science Enterprise. The
Enterprise is dedicated to understanding the Earth as an
integrated system and applying Earth System Science to improve
prediction of climate, weather, and natural hazards using the
unique vantage point of space. The California Institute of
Technology, Pasadena, manages JPL for NASA.

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